TAXONOMY

How to label all life, and lichen?

I am an inveterate binocular user, rarely going anywhere without a pair. They are useful not merely for enjoying scenery, and birds and mammals in their natural settings, but also for the odd peregrine falcon perched incongruously atop an urban high-rise, or for admiring architectural detail in close-up at castles and cathedrals.

Flip them round, though, look through the objective lens and bring the eyepiece up close to something . . . and suddenly you have a portable makeshift microscope ready to reveal a whole new world. Of course they are no match for the high-powered, desk-mounted microscopes found in laboratories, but then they are far more portable and far less expensive.

Taxonomy is the science of formulating our ever-developing understanding of the complex relationships between living organisms. As technology has advanced, so too has our understanding of these relationships. The development of the microscope, giving natural scientists their first-ever view into a world invisible to the naked eye, revolutionized taxonomy.

It was Swedish botanist Carolus Linnaeus (1707-78) who founded the now internationally recognized binomial nomenclature for all life forms, in which the Latinized genus name precedes the species name — as in Falco peregrinus, for that incongruously perched bird of prey — in a form usable regardless of the vernacular name.

Linnaeus’s preference for observation with the naked eye, however, meant that his taxonomic system, based on a simple division of all living organisms into two kingdoms — the Animals and Plants — was already out of date when he was formulating it. By then, others were already seeing things beneath the lens that defied such simple classification.

The discovery — made by microscopists — that there were free-living, single-celled organisms, and the subsequent discovery in the 1830s that plant and animal tissues were both made up of individual cells, with each one having a nucleus, pushed Linnaeus’s two-kingdom view to breaking point.

While animals and plants both share commonalities in their makeup, many other organisms are very different indeed, and in modern taxonomy it is these differences that are crucial.

Despite the growing body of observations of different forms, however, it remained easier to apply the shoehorn approach to taxonomy: squeeze only superficially similar forms into the same kingdom (thus the fungi remained among the plants), rather than take the bull by the horns and shake up both the established hierarchies and their supporters.

Building on evidence developed during the late 19th and early 20th centuries, in the 1950s the American biologist Robert Whittaker of Cornell University proposed a bold revision to the taxonomy of life, basing it not on two kingdoms, but on five: Animalia, Plantae, Fungi, Proctistae (multi-cellular organisms that don’t fit into any of the preceding three kingdoms) and Monera (single-celled prokaryotes). Whittaker gave important recognition to the fact that the fundamentally different fungi could no longer be thrown in with the plants, and that single-celled organisms without a nucleus are profoundly different from organisms with nucleated cells.

Though the five-kingdom systematics hold sway today, it was conceptualized before the advances of modern molecular biology, a science that has unleashed further surprises, revealing, for example, profound molecular differences within the bacteria. Now two completely distinct groups are recognizable, the Archaebacteria and the Eubacteria.

As a result, arching over the Linnaean Kingdoms, we now have three Domains: the Archaea (Archaebacteria); the Bacteria (Eubacteria); and the Eucarya (containing all of the nucleated cellular organisms including the kingdoms of fungi, plants and animals). As further molecular evidence accumulates, our understanding of systematics will no doubt change, adding more kingdoms to the current five as we better recognize the similarities and differences between organisms at the molecular level.

A revelation for me was that the three great kingdoms I had accepted as representing the elemental divisions of life — into the plants, animals and fungi — are, in fact, quite closely related, having shared a common ancestor not much more than 1 billion years ago. Fungi, it now seems, are more closely related to animals than either group are to plants.

But then just when you think you understand something, along comes new information to shake up your beliefs. Raise your hands if you thought, for example, that lichens were a distinct group of organisms.

Those crusty, flaky things that coat rocks, old tombstones, memorials and the walls and even roofs of old houses, farm buildings and churches, the trunks of certain trees, and the branches of those in foggy areas, are typically taken for granted as part of the structures on which they grow, and are for the most part ignored.

Pause to take a closer look though. Take out a hand lens — or flip over your binoculars — and a whole new world of color and texture is revealed. Their apparent similarities suggest they are a group of related species of organisms, yet in reality “lichen” is a term that represents an alternative lifestyle choice — as shocking a revelation to some, perhaps, as the concept of premarital cohabitation in the 1960s.

A lichen’s thallus (in other words the plant body, showing no clear distinction of roots, stem or leaves) represents an extremely alternative form of cohabitation, a cross-species indeed. Two quite different organisms — a fungus and an alga or cyanobacteria — grow together as one, with the fungus making up the bulk of the thallus and with the algal cells buried within it. More of this bizarre lifestyle choice in my next column.